A regional law-enforcement effort has been in place to educate vehicle-rental businesses on how to spot potential terrorists — but the effort could not have stopped Tuesday’s carnage, police sources said.

Law-enforcement agents have talked to some 20,000 people in the private rental business in New York City and nearby towns as part of the SHIELD program over the past two years, since ISIS urged lone wolves to attack soft targets with motor vehicles, according to the NYPD.

But Sayfullo Saipov, the suspect in Tuesday’s rampage, had previously worked as a commercial truck driver and had a license in good standing, so he would not have raised any red flags, sources said.

“We’re constantly talking to people and retalking to people but there was no way to stop someone like this,” one law-
enforcement source said. “But we can’t stop everything.”

John Miller, the NYPD’s deputy commissioner of intelligence and counterterrorism, told reporters Tuesday that local rental merchants have been urged to keep an eye open.

“So the industry has had a high level of awareness on this matter from the NYPD,” he said.

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But there are only so many questions even the most vigilant business person can ask.

“Anybody that has a driver’s license can get behind the wheel of any vehicle and decide they want to run people down. How can we stop that?” a high-ranking police source said.

“You’re asking private commercial companies to do our job . . . to lose cash revenue . . . or you’re asking them to profile certain types of people, which they absolutely are not going to do.”

The emphasis on trucks as potential murder weapons is particularly on the minds of local authorities at this time of year, as ISIS has said the Thanksgiving parade would be an ideal target.

“The method of such an attack is that a vehicle is plunged at a high speed into a large congregation of [nonbelievers], smashing their bodies with the vehicle’s strong outer frame,” ISIS said in its English-language magazine Rumiyah last fall.